STRUCTURE OF THE MARTIAN IONOSPHERE: OBSERVATIONS OF SUPRATHERMAL ELECTRONS BY MAVEN SWEA
Abstract:The Solar Wind Electron Analyzer (SWEA) onboard the MAVEN spacecraft measures the energy and angular distributions of suprathermal (3 eV to 4.6 keV) electrons in the Mars environment. SWEA's energy resolution (ΔE/E = 17%), large field of view (80% of the sky), and 2-sec (~8-km) measurement cadence provide a detailed picture of ionospheric structure and magnetic field topology to altitudes as low as 125 km. During the first months of the mission, periapsis sampled northern latitudes from 30 to 75 degrees and altitudes down to 150 km, including the dawn and dusk terminators, the day and night hemispheres, and regions with and without significant crustal magnetic sources. The interface between the ionosphere and the overlying magnetosheath is structured and variable. When present, crustal magnetic sources can trap ionospheric plasma and significantly influence this structure.
On the night hemisphere, suprathermal electrons are consistently observed down to the lowest altitudes reached by the spacecraft. Above ~200 km altitude, two sources of suprathermal electrons are observed: transport of ionospheric plasma from the day side and precipitation of electrons from the magnetotail. From 200 to 125 km altitude, the suprathermal electron density drops by more than a factor of 1000, as these electrons interact with the neutral atmosphere. At the lowest altitudes, the residual population peaks at 7 eV (see figure), where there is a local minimum in the total cross section for interaction with CO2. On the day hemisphere, the spacecraft reached altitudes as low as 140 km near the sub-solar point, close to where the photoionization and electron impact ionization rates are expected to peak. A local maximum in the 5-10-eV photoelectron flux is consistently observed at ~150 km, which is consistent with passage through this ionization rate peak.